Phase-change materials exist in a crystalline state or an amorphous state according to temperature. A phase-change material has a lower resistance and a more ordered atomic arrangement in a crystalline state than in an amorphous state. A phase-change material can be reversibly transformed from the crystalline state to the amorphous state based on an operating temperature. Such characteristics, that is, reversible phase change and different resistances of different states, are applied to newly proposed electronic devices, a new type of nonvolatile memory devices, phase-change random access memory (PRAM) devices. A resistance of a PRAM may vary based on a state (e.g., crystalline, amorphous, etc.) of a phase-change material included therein.
Various types of phase-change material can be used for memory devices, the most commonly used phase change materials are ternary composition of chalcogenides of group 14 and group 15 elements, such as germanium-antimony-tellurium compounds, commonly abbreviated as GST. The solid phases of GST can rapidly change from crystalline to amorphous or vise versa upon heating and cooling cycles. The amorphous GST has relatively higher electric resistance and the crystalline GST has relatively lower electric resistance.
Currently, Physical Vapor Deposition (PVD) processes, or spattering, are used in the manufacture of re-writable optical disks to coat a thin layer of phase change material on the plastic substrates. However, the PVD processes are not suitable for electronic devices due to film growth control and film properties. To make PRAM, Chemical Vapor Deposition (CVD), or Atomic Layer Deposition (ALD) techniques are used to deposit a thin film of GST on the substrate of silicon. The development of phase-change memory devices raises the need for ALD/CVD processes with proper precursors for low temperature deposition.